The field of the invention pertains to vertical extruders for extruding plastic parisons in conjunction with almost all types of blow molding machines including multistation indexable rotary table blow molding machines for the molding of plastic bottles and the like. In particular, the invention pertains to improvements in the vertical extruder to provide a smoother path for the extruded material and a shortened extruder length.
Vertical extruders commonly comprise a rotatable extrusion screw located within a hollow tube. The extrusion die for forming the parison is located at the lower end of the extrusion tube and includes a fixed or vertically movable central mandrel to produce a hollow plastic tube or parison. At the upper end of the extrusion tube and rotatable extrusion screw is an inlet for feeding raw plastic material, typically in pellet form, to the extrusion screw. The extruder is usually heated. The rotational motion of the screw in conjunction with the heat plasticizes the pellets into a highly viscous fluid material which is thereupon squeezed out through the extrusion die to form the parison.
The typical parison extrusion screw and extrusion tube is a substantially tall device of relatively small diameter exceeding a length to diameter ratio of twenty. To support and move the central mandrel, lateral support and actuation means extend from the inside wall of the extrusion tube to the mandrel at a location above the extrusion die and below the extrusion screw. Unfortunately, such lateral support and actuation means, sometimes called a spider, interferes with the movement of the highly viscous plastic material from the screw toward the extrusion die. The spider adds substantial resistance to the flow of the material and with some plastic materials can cause unsightly burn spots in the products manufactured by the blow molding machine. The burn spots are caused by particles of resin which are caught on the entrance or exit surfaces of the spider and therefore fail to flow smoothly with the resin.
Typically, the spider in the passageway between the lower end of the extrusion screw and the exit of the the extrusion die frictionally eliminates the rotating component of the viscous plastic prior to exit from the extrusion die. Thus, the rotating component of the viscous plastic is reduced to a negligible amount in the parison as it exits the die to prevent malformation of the parison as the mold cavity of the blow molding machine closes about the parison.